Ground Motion Occurrence Rates for Scenario Spectra

It is common practice for probabilistic seismic hazard analysis (PSHA) to use the uniform hazard spectra (UHS) to describe the ground motion. A short-coming of UHS is that it represents an envelope of many different earthquakes that control the hazard at different spectral periods. As an alternative, the UHS can be broken into a suite was developed by Baker and Cornell (2006), in which conditional mean spectra (CMS) are developed that represent realistic earthquake scenarios given a specified spectral acceleration at a single period. Using only the CMS as scenarios is too restrictive and does not provide enough scenarios to reproduce the hazard. The concept of the CMS is expanded to produce three scenario spectra for each CMS. The three scenarios represent the mean (the CMS) and two lower fractiles of the conditional spectra. Using these three scenario spectra for three difference spectral periods (0.2, 0.5, and 2.0 sec) and four different return periods (250, 500, 1000, and 2500 years) results in 36 scenario spectra. Rates for these 36 representative scenarios can be derived that approximate the hazard curves at all three spectral periods simultaneously. The advantage of the new approach over the CMS approach is that it provides rates of occurrence of the scenario spectra in addition to providing realistic scenario spectra. These scenario spectra with their associated rates of occurrence can be used in seismic risk calculations for estimating the probability of structural performance